Oh, all sorts of reasons, Louise.
For some kids it's down to laziness on the part of the parents who refuse to discipline their kids. I know plenty of parents who simply ignore their children when they misbehave or pretend not to notice their behaviour.
For others it's parents spoiling their children - you know, the "I don't want them to be unhappy so I give them what they want" etc - both material things and attention.
Also, I think a lot of parents "overcompensate" for the social situation they find themselves in - eg both parents working, or split up/second families, which they feel has a negative effect on the children.
And our communities have changed - whereas it used to be acceptable that if you were cheeky to your next-door-neighbour/postie/shopkeeper down the road you would get a clip round the ear or a telling off (and your parents would give you a row too) - this doesn't happen any more - in fact people are scared of telling off kids that are not their own. There is no "collective responsibility" any more.
People often compliment us on the good behaviour of our dds, but these are the same people who criticise us for being "hard" on them - er, bit of a correlation here...